A typical motor vehicle is generally characterized as comprising an unsprung mass and a sprung mass. The unsprung mass generally consists of all of the parts of the vehicle not supported by the vehicle suspension system such as the tire/wheel assembly, steering knuckles, brakes and axles. The sprung mass, conversely is all of the parts of the vehicle supported by the vehicle suspension system. The unsprung mass can be susceptible to disturbances and vibration from a variety of sources such as worn joints, misalignment of the wheel, brake drag, irregular tire wear, etc. Because vehicular tires support the sprung mass of a vehicle on a road surface and such tires are resilient, any irregularities in the uniformity or dimensions of the tire, any dimensional irregularities in the wheel rim, and/or any dynamic imbalance or misalignment of the tire/wheel assembly will cause disturbances and vibrations to be transmitted to the sprung mass of the vehicle thereby producing an undesirable or rough vehicle ride, as well as reducing handling and stability characteristics of the vehicle. Severe vibration can result in dangerous conditions such as wheel tramp or hop and wheel shimmy (shaking side-to-side).
It is now standard practice to reduce these adverse vibrational effects by balancing the wheel rim and tire assembly by using a balance machine and clip-on lead weights or lead tape weights. The lead balance weights are placed on the rim flange of the wheel and clamped in place in a proper position, or adhered to the wheel in the case of tape weights, as directed by the balancing machine. In general terms, balance is the uniform distribution of mass about an axis of rotation, where the center of gravity is in the same location as the center of rotation. A balanced tire/wheel assembly is one where the mass of the tire/wheel assembly mounted on the vehicle's axle is uniformly distributed around the axle. Balancing is an improvement and will reduce the vibration of the tire/wheel assembly in comparison to an unbalanced tire/wheel assembly.
Another method of balancing is provided by balancing rings. Balancing rings typically comprise a 360 degree annular tube partially filled with weights (typically less than 50% of the tube) in combination with a damping fluid which typically fills the remainder of the tube. The tubes are typically attached adjacent the wheel flange. The most common commercial balancing ring is produced by Centramatic, which uses steel shot in oil. The balance ring works by making use of centrifugal force to distribute the steel shot inside the tube to compensate for dynamic tire balance. As the tire/wheel assembly rotates, the steel shot in the tube will flow away from a heavy spot of the tire until such time as the out of balance situation is corrected. The centrifugal force holds the weight against the outside of the balancing tube. The key to balancing rings is that the weight in the ring must be able to move within the tube to counter the heavy spot of the tire. Therefore a balance ring does not have a constant mass about its circumference, either at rest or in use on a tire/wheel assembly.
However, even perfect balancing of the tire/wheel assembly does not necessarily mean that the tire will roll smoothly. Even a perfectly balanced tire can have severe vibrations due to non-uniformities in the tire which result in unequal forces within the tire footprint.
A level of non-uniformity is inherent in all tires. In the art of manufacturing pneumatic tires, rubber flow in the mold or minor differences in the dimensions of the belts, beads, liners, treads, plies of rubberized cords or the like, sometimes cause non-uniformities in the final tire. When non-uniformities are of sufficient magnitude, they will cause force variations on a surface, such as a road, against which the tires roll and thereby produce vibrational and acoustical disturbances in the vehicle upon which the tires are mounted. Regardless of the cause of the force variations, when such variations exceed the acceptable minimum level, the ride of a vehicle utilizing such tires will be adversely affected.
While gains have been made in the ability of the average tire shop to diagnose, measure, and correct vibration of a tire/wheel assembly due to imbalance, run out, and non-uniformity force variations, there remains a need in the art to provide stability to the unsprung mass of the vehicle to combat tires that may have excessive force variations due to non-uniformity. It is unrealistic to believe or expect that all of the tires that do not meet the radial force limitations of the vehicle will not find their way onto a vehicle. Accordingly, it would be an advantage to provide a uniformity and stabilizing system to help improve the performance of the population of tire/wheel assemblies as a whole.